Space Elevator: Expert Q&A

On January 16, 2007, physicist Bradley Edwards answered questions about the concept of space elevators.

Monday, January 1, 2007 Nova

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Q: Archimedes was supposed to have said, "Give me a place to stand on, and I can move the Earth." This seems like a similar predicament: "Give me a hook to hang my tether on, and I'll build a space elevator." Where will the hook be? Prasad Golla, Plano, Texas

Brad Edwards: The "hook" will be a satellite in high orbit. Due to the rotation of the Earth, the satellite and the upper half of the ribbon are thrown outward away from Earth. An easy way to think about this is to consider a ball on the end of a string that you spin around your head—the string sticks straight out with the ball on the end. In the case of space elevators, Earth does the rotating and the elevator is held in place.

Q: This whole project appears to hang on carbon nanotube fibers being developed with sufficient strength to support the "cables." How close is present development to achieving this goal? John Harraway, Agios Nikolaos, Crete, Greece

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Edwards: Carbon nanotubes were measured with sufficient strength a few years ago. The issue has been to combine them into a long thread and then a ribbon. Spun threads with nearly the performance we need—withstanding 50 GPa (gigapascals) versus 63 GPa to 120 GPa of tension—were made at the Los Alamos National Laboratory not long ago. We currently expect the material to be available for use within the next couple years.

Q: How can your space elevator possibly be protected from sabotage or terrorist attacks? Talk about a tempting target! Bill Pitts, Mississippi

Edwards: Nothing can be completely protected, although the elevator is out in the middle of the ocean and far away from everything. It is a challenge to get to and difficult to "sneak up" on. The other issue is that an attack on the elevator would cause a financial impact to a company, but minimal loss of life in the scenario in which we plan to use it. Everything, including the elevator, is, of course, a potential terrorist target, but we suspect we may be lower on the list.

Q: How much material could be taken onboard the space elevator, and what kind of containers would the items be transported in? Anonymous

Edwards: The first elevator we have designed would carry 13 tons every three days or five tons every day. The containers would depend on the cargo. Satellites would have minimal containers, while supplies or people would need to be in airtight containers to protect them from the space environment. Anytime something goes to space, it will need to be protected. That said, with the space elevator, the containers wouldn't need to be built to survive the violent forces experienced with rocket launches.

Q: Would the elevator burn up on its way down or freeze on its way up? How would the cable resist the temperature changes? Lars, Eleva, Wisconsin

Edwards: Actually, the temperature doesn't change that much as you ascend the elevator. Temperature changes are more related to going in and out of the Earth's shadow (day and night). In space, it is also the case that the sun warms one side and space cools the opposite side of any container. Good insulation and thermal conduction are required.

Q: What is harder, getting $10 billion to start, or solving the technical problem of spinning carbon nanotubes into a ribbon? Tony, Issaquah, Washington

Edwards: No question—getting $10 billion. We are almost done with the second.

Q: Beyond the technical challenges to be addressed, what are the greatest political obstacles to building a space elevator? Neil Kauffman, Kirkland, Washington

Edwards: There are a few, and we have yet to find out which will be the most challenging. There will be countries upset with others for constructing it, there will be legal issues concerning right of way for spacecraft in orbit, and then there will be those nations, organizations, or individuals who just want to take a piece.

Q: I keep hearing that there is a time frame of about 15 years before we have a working space elevator. I believe this time could be shortened if the country was mobilized behind this concept the way we were in the '60s for the Apollo program. Is the President of the United States even aware of how important the elevator is to the future of space travel? Jane Montgomery, Fort Worth, Texas

Edwards: Yes. With a mobilized effort, we could have the elevator up quickly, even sooner than in 15 years. There are several options on how to do this, including the U.S. government getting involved. I am sure the President knows of this project, but I am sure that it is not seen as a priority or a good political move—it could be risky and require more spending. Our society focuses on the "now," and the elevator is critical for our future—a poor match.

Edwards: Very heavy payloads will pull the elevator out of orbit, so we will be limited to the specified mass the elevator is designed for. In our case, the first elevator is designed for a maximum of 13 tons, although larger elevators with higher payload capacities can be built.

Q: I think the space elevator is a terrific idea, and I understand how it works, but how would you maintain stability above, say, 1,000 feet? Couldn't one severe storm start the apparatus twisting and swaying, bringing the structure down? Jim Suss, Marietta, Georgia

Edwards: Due to the dynamics, the elevator is held extremely tight and will be fairly rigid. Storms and winds will blow it around and move it. We will need to deal with twisting and oscillations, but one thing to remember is that the frequency of this system is seven hours, a very slow oscillation. The other aspect is that the anchor will be located in the eastern equatorial Pacific Ocean at a place devoid of major storms, wind, or lightning, just north of the doldrums.

Q: Would the space elevators have to be placed exactly on the equatorial line, like a satellite in a geosynchronous orbit, to maximize the centripetal force of the Earth's rotation to stiffen the elevator cable? Or would they work equally well anywhere on the surface of the Earth? What would be the "orbital physics 101" considerations here? Attila Gyuris, Lake Matthews, California

Edwards: The elevator can be placed anywhere within 20 degrees of the equator due to the dynamics, but the performance is best at the equator. As you get off the equator, the ribbon goes up at an angle and eventually is lying close to the ground and is unusable.

Q: Would the space elevator be something open to the public, or just to private or government organizations, e.g., NASA using it to repair the space station? Richard, Colonial Heights, Virginia

Edwards: It really depends on who builds it, but we are hoping it will open space to everyone.

Q: What are the dangers to aviation with regard to the cable? Anonymous

Edwards: We will need to have airplanes stay clear of the ribbon. This shouldn't be a problem since the anchor is located hundreds of miles from any air routes.

Q: How will we get the rest of the way to the moon or to Mars from the top of a 22,000-mile-long space elevator? Cjarles Pinter, Germantown, Tennessee

Edwards: The ribbon itself will be close to 100,000 kilometers (or 62,000 miles) long, and the upper end will be going faster than orbital velocity. Anything released from the upper end will be thrown fast enough to reach the moon and Mars, as well as other locations. No rockets will be needed, except for directional corrections.

Q: How long will it take to construct the elevator? Anonymous

Edwards: The elevator should take about 10 years to build once a dedicated effort has begun.

Q: How big can the elevator get? What I mean is, what is the maximum amount of cargo that a theoretical elevator could take up into orbit at once? Anonymous

Edwards: An upper limit is difficult to state, but we have already considered possible systems that could carry up to 1,000 tons. These are very large and require massive engineering but should be viable.

Q: It's been said that the trip to space will take five to seven days. Is there any way of speeding that up to, say, a day or two? Curran, Grand Rapids, Michigan

Edwards: Yes, the first one would take a week to geosynchronous orbit, but later elevators will be improved. Eventually we would expect it to take a day or so, but that has yet to be developed.

Q: While I was teaching at the Air Force Academy, a student and I modeled the likelihood of satellite impacts with the ribbon from just Low Earth Orbit (LEO) satellites. Our conclusion was that the ribbon would probably have to move several times per week. This is easily done with a mobile oil platform, but can you accurately predict the exact position of the ribbon at LEO altitudes following motion of the oil platform (especially in rough seas)? Tom Joslyn, Colorado Springs, Colorado

Edwards: We have gone through the dynamics and have a pretty good idea of how it will act, but the plan is to track the ribbon with radar and lidar so we know exactly where it is.

Q: Could more than one load be lifted at a particular time, such as a "train" of individual climbers? Doug Dexheimer, Overland Park, Kansas

Edwards: Yes, we plan to have up to six on at a time. Once a payload is out of Earth's gravity, another can be started.

Q: If the cable weighs one-tenth of a pound per foot, its total weight would be over ten million pounds. What would the tension distribution be in the cable even without a payload riding up or down? Bill Tobey, Littleton, Colorado

Edwards: This is actually where pretty much all the tension comes from, and it would be roughly 50GPa depending on the ribbon design. The tension would be designed to be uniform along the ribbon.

Q: Several science fiction authors have written on this subject. Was Arthur C. Clarke the first? Who has come closest to current theory? Mitch Burte, Andover, Massachusetts

Edwards: I believe Clarke was the first in a novel, and his most recent novel with Stephen Baxter uses the newest data. They did a very good and accurate job.